There is known an art in which a foamed or porous metal is produced by adding a foaming agent to a molten or powdered metal and gasifying the foaming agent by, for example, heating to form numerous pores in the metal. In the narrow senses of the words, the foamed metal containing gas in its numerous pores differs from the porous metal emitting such gas, but since they are equal in having numerous pores, they are herein called by a combined name as a foamed/porous metal.
A method of manufacturing a foamed/porous metal is proposed in, for example, Japanese Patent No. 2,898,437 entitled “Method of Manufacturing a Foaming Metallic Body”, and stating specific examples of a foaming agent, such as “0.2% by weight of titanium hydride” and “sodium hydrogen carbonate”. The use of titanium hydride or sodium hydrogen carbonate containing hydrogen having a high reducing power is usual for foaming aluminum having a high affinity for oxygen. The above patent includes the statement:“A metallic body floats in water. There are formed pores distributed uniformly through the metallic body and having nearly the same size. The size of the pores is controlled by the length of time during which bubbles expand in the metal in a foaming process.”
The invention according to the above U.S. Pat. No. 2,898,437 is aimed at manufacturing merely a metallic body floating in water. A recent requirement is, however, for a structural body to have a part serving both as a reinforcing member and a porous metal to realize a reduction in weight, and the prior art described above is insufficient in strength for satisfying such requirement.